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I'm currently planning on creating a sous-vide setup using an induction cooktop and a PID controller. I will be an Arduino micro-controller setup similar to this openschemes.com-manual control of induction cooktop

Most sous-vide setups use circulators that circulate and mix the water in the bath to keep uniform temperatures throughout the bath. I was wondering if a pump is required if I'll be using a pot on top of an induction cooktop. Since the heat source is at the bottom, won't the water rise naturally through convection and keep a constant temperature throughout the bath?

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Wait, you found an induction cooktop which can be turned on and off by just giving it current? The only models I have seen require somebody to touch the "power" button and then set the temperature dial, this is why most such setups use a different method of heating. –  rumtscho Sep 28 '12 at 20:27
    
@rumtscho so I left out a few details. I'll be opening it up and hooking up an Arduino to the control wires of the cooktop. It'll always be on and I'm going to try just lowering and raising the voltage as needed. But your right, I also couldn't really find a cheap alternative with an analog control. Here is a link to the induction hack: openschemes.com/2011/04/28/… –  Tomek Sep 28 '12 at 20:54
    
Well, it can be done, the Sous Vide Supreme manages it, though from reading their blog, they are fairly sophisticated, with multiple heating elements and thermometers (as I recall, it's been some time). –  Ronald Pottol Dec 4 '12 at 23:29

2 Answers 2

up vote 2 down vote accepted

What you will be dealing with is called stratification. Given a reasonable volume of water the difference can be quite remarkable. A one metre height of water can stratify water from 20°C to 95°C as long as the water is not disturbed and heated gently, even if heated from the bottom

The simple solution is to regularly stir the water, say once every five minutes. This would be OK for items only requiring an hour in sous-vide, but for much longer this can get rather tedious

If you are targeting temperatures in the 40°C to 65°C range a small aquarium pump would suffice to stir the water. Arrange the intake tube nearly floating on top, and the output tube weighted on to the bottom. At a pinch an aquarium bubbler would help significantly too. Both these devices will cool the water somewhat, but a stove top heater should be able to keep up

Example: Using a small bubbler in a large, well-insulated chest (Rubbermaid cooler, Esky, chilly bin etc.) with around 10 l of water at 60°C, keeps the temperature within 1°C from top to bottom. Heat loss is around 1°C per 30 minutes. By adding about 0.5 l of 95°C water every 30 minutes it keeps the temperature constant over a few hours

To get precise temperature control within ±0.5°C which some sous-vide recipes recommend, you will need a PID controller. For general home use with temperature control or ±3°C you could get away with a stove top heater, with accurate power control and a thermometer control system. PID is not that hard, so you might as well add that to your controller too

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Would an induction cooktop be considered heated gently? I tested out the cooktop last night and it was able to boil about half a liter of water in 2 minutes or so. I know something like a slow cooker would count as a gentle heating, but what effect does the fast heating of an induction cooktop have on stratification? –  Tomek Oct 1 '12 at 15:41
    
@Tomek The Arduino controller hack is using fully proportion control (PWM), so it should be able to go from 1% to 100% power without issue? –  TFD Oct 1 '12 at 19:49

I made a sous-vide setup that uses a slow cooker and no pump.

I was able to observe as much as a 5 degree temperature gradient from the bottom to the top of the cooker. A big part of the problem was that my target food almost fills the cooker and impedes convection. I didn't measure the temperature gradient with an empty bath.

My setup worked passably well as long as my food was small and my thermometer was positioned at the same depth. I didn't enjoy the precision that commercial products have.

Obviously your setup will have a lot more power than mine. If your bath is much bigger than your target food you would have less of a temperature gradient than I did.
If your bath is much larger than your target food then convection will give you enough water circulation. Even a 5 degree drift will let you do some interesting things, however, It is one more variable to keep track of.

I was not satisfied with my setup and my next attempt will have a stronger heater as well as a circulation pump. Water pumps are cheap.

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Thanks! I also have a 3 quart slow cooker that I was planning on experimenting with. What did your process look like? Would you heat the slow-cooker to the desired temp and then drop in the food? After dropping in the food, did you do anything after that or just wait? From my understanding the plate for a slow cooker is on the bottom, but the stoneware still creates a bit of space in between the cooker and itself and so the air around the stoneware is also warm? Not as warm as the bottom but I would imagine it still provides heat not only from the bottom but form the sides as well. –  Tomek Sep 28 '12 at 20:59
    
The slow cooker was slow enough that I would preheat the cooker and the water before placing the food into it. It would take a bit of time for the food to come to temperature. The sides do provide heat. I haven't measured their temp compared to the bottom. That would be interesting. –  Sobachatina Sep 28 '12 at 21:02
    
Incidentally- it is usually considered good form to wait a day or two before accepting an answer. @rfusca might chime in here with a brilliant answer about his setup. –  Sobachatina Sep 28 '12 at 21:03

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